H. C. Bryant

Atomic Physics with Relativistic Beams

The availability of a beam of H- ions with near-luminal velocity has enabled us to investigate photon-atom interactions at photon energies and electric field strengths presently beyond the scope of ordinary laboratory techniques. See Fig. 1. We have surveyed the entire photodetachment spectrum of H- from threshold for single electron detachment to well above the threshold for two-electron detachment. Embedded in the one-electron photodetachment continuum of H- are a series of doubly excited states, termed “resonances” since they are unstable against autodetachment into the continuum. The two most striking of these lie near the threshold for the production of H° (n = 2): a very narrow state just below the threshold, the “Feshbach” resonance, and a broader state lying just above the n = 2 threshold, the “shape” resonance. By means of Stark mixing, we have been able to use these 1P states to uncover nearby 1S and 1D states. Just below the threshold for H° (n = 3) production, we have found a prominent dip in the photodetachment cross section. This “Feshbach”-type state is accompanied by a narrower recursion lying between it and the n = 3 threshold. The single electron Open image in new window Fig. 40. We are studying a System moving with 84% the speed of light. Time is dilated and lengths are contracted by 1.853. photodetachment total cross section above n = 3 shows no further distinctive features upon examination with our current resolution, although there is some evidence for structure below n = 4.

Spectra from multiphoton electron detachment of H

New data are reported on the multiphoton detachment process in a fast beam of H− ions. The angle-tuned relativistic Doppler shift is used to vary the photon energy of a focused (~10 GW/cm2) 10.6-μm CO2 laser beam from ~0.05 to ~0.4 eV in the rest frame (CM frame) of the fast ions. The ions are produced at 800 MeV (β = v/c = 0.84) by the Los Alamos Meson Physics Facility linear accelerator at Los Alamos and experience ~1-psec pulses in the CM frame as they cross the laser beam focus. Peaks in the detachment signal corresponding to each order for two- to six-photon processes are observed. At modest laser intensity in the gigawatt-per-square-centimeter range, observed shifts of the apparent two-photon threshold are found to be not more than 30–50% of the expected maximum shift, based on the value of the ponderomotive potential. Experimental uncertainties are due mainly to imprecise knowledge of the maximum laser intensity. The data analysis and modeling of the expected threshold shape experiments are continuing.

Measurement and reduction of momentum spread in the LAMPF linac beam

The use of laser spectroscopy for the determination of momentum spread and energy of a relativistic H− beam is discussed, and the results are presented of a successful attempt to reduce the momentum spread δpp by a factor of 5 from its initial value of approximately 5 × 10−4.

Measurement of H/sup 0/ excited states produced by foil stripping of 800-MeV H/sup -/ ions

Foil stripping of H/sup -/ directly to H/sup +/ is being considered for proton injection in the next generation of high-current proton storage rings. This technique can result in significant losses because excited states of H/sup 0/, which are also produced in the foil, are field stripped in the downstream bending magnets. Without due care in the injection system design, many of the resulting protons will be outside the acceptance of the storage ring and will be quickly lost. We measured the production of such H/sup 0/ excited states at the LAMPF High Resolution Atomic Beam Facility. An 800-MeV H/sup -/ beam was passed through carbon foils of thicknesses 70, 100, 200, and 300 /spl mu/g/cm/sup 2/ and the excited states were analyzed by a special magnet downstream of the foil. The magnet had a linear field gradient so that the trajectories of the outgoing protons could be used to reconstruct the field values at which the various H/sup 0/ stripped. We found that about 1% of the H/sup 0/ emerge in excited states which can be stripped to protons by ring-bending magnets.<<ETX>>

Excitation of atoms passing through a TEM 00 Gaussian laser beam at relativistic velocities

The spectral distribution seen by an atom crossing a monochromatic Gaussian TEM00 beam with a relativistic speed normal to the direction of propagation of the beam is shown to be independent of the distance between the atom’s path and the position of the beam’s minimum waist. The dependence of the transition probability on the beam coordinates is also given. Finally, it is shown that the spectral width can be viewed as being due to the angular distribution of the plane waves of which the Gaussian TEM00 beam is composed.

Photodetachment of Relativistic Ions

A series of fundamental laser ion beam experiments has been made feasible by the high-quality, relativistic (ß = 0.842) H- ion beam available at the Clinton P. Anderson Meson Physics Facility (LAMPF). The relativistic Doppler shift of the light from an ordinary ultraviolet laser provides what is, in effect, a continuously tunable vacuum-ultraviolet laser in the rest frame of the moving ions. The Lorentz transformation of a modest laboratory magnetic field provides an electric field of several megavolts/centimeter. The latest results of our photo-detachment work with H- beams and our spectroscopic work with H0 beams are presented. Our plans for future work are discussed.

Photodetachment of the H− ion

Abstract Detachment of electrons from the H − ion is investigated with an experimental technique whereby an H − beam moving at a relativistic velocity (2.5 × 10 10 cm/s) is intersected with a fixed frequency laser. The Doppler effect allows systematic variation of the center-of-mass (CM) photon energy over a wide range (factor of 10) by simply adjusting the angle between the ion and laser beams. The focused output from a pulsed, linearly polarized, CO 2 TEA laser operating at 10.6 μm, with peak intensities on the order of 10 GW/cm 2 , was used to examine the multiphoton absorption process in H −2 . The fourth harmonic (266 nm) of a Nd:YAG laser was used to investigate some of the doubly-excited state resonances in H − . In the multiphoton absorption work, electron detachment was observed at photon energies where as few as 2 and as many as 8 photons are required to get above the 1-electron detachment threshold (EDT) of H − (0.754 eV). Electron yield vs photon energy plots exhibit structure that is laser intensity dependent. Electron yield vs laser pulse energy data was obtained at a few selected CM wavelengths and laser pulse energies. In the single-photon UV laser work, numerous resonances within the H − photodetachment continuum corresponding to one-photon two-electron excitation processes were observed. The doubly-excited resonances appear to be of the Feshbach type. A simple, semi-empirical recursion formula predicts the resonance energy levels. The experimental techniques described here can be used to accurately determine accelerator beam and ion source parameters such as beam energy, energy-spread, and ion density spatial distribution.

Observations of excited H° atoms produced by relativistic H− ions in carbon foils

Studies of the production of H° its various states by the passage of relativistic H− ions through thin foils are under way at the linear accelerator at Los Alamos. Laser excitation and selective field ionization are used to isolate specific states. We present evidence that the excitation of Rydberg states by a foil at energies up to 800 meV proceeds in a stepwise manner.

Laser spectroscopy with relativistic beams

Relativistic H‐beams with an energy of 650 MeV are combined with pulsed ultraviolet lasers to study photodetachment cross sections in externally applied electric fields. (AIP)

Laser Spectroscopy of Relativistic Beams of H- and H: Observation of e-Detachment from H- by Multiphoton Absorption

Publisher Summary This chapter discusses laser spectroscopy of relativistic beams of H- and H by multiphoton absorption. Multiphoton detachment (MPD) takes place with lower energy photons than typical multiphoton ionization (MPI) with neutrals. The lack of intermediate-states simplifies the calculations as does the fact that the final-state interaction is of short range rather than Coulomb: H- may provide a particularly clean test of MPI/MPD theory. Large changes in the MPD rate can be expected when a large static electric field is applied in addition to a laser field. In the experiment described in the chapter, detached electrons were detected with high efficiency and low background by the electron spectrometer; fast η atoms were detected by a downstream scintillator. Observed MPD rates with the smoothing tube were somewhat smaller than without it, as expected, but at the highest intensities, partial depletion saturation was observed of at least the 3-photon threshold detachment on the ∼1 psec scale of the H- transit time through the laser focus.